Due to heat resistance, polyimide-based materials are widely used in adhesives, sheet materials, sealants and the like. In general, polyimide is dissolved in a solvent to form a varnish, which is applied to a semiconductor device or the like directly or via an insulating film and cured to form a protective film of polyimide resin. This is followed by encapsulation with a molding material, typically epoxy resin. The resulting package, in which components including the chip, substrate and encapsulant have different coefficients of expansion, suffer thermal stresses during subsequent steps such as thermal cycling and solder reflow, inducing chip cracks or thermal degradation. It is thus required to endow a polyimide resin protective film with a sufficiently low modulus of elasticity to absorb such stresses. One approach for reducing the modulus of elasticity is by incorporating siloxane linkages into the polyimide resin structure. The problems of this approach are that the cured film has a lower glass transition temperature (Tg) and the resin has degraded heat resistance. For example, JP-A 5-009254 and JP-A 6-116517 disclose siloxane-modified polyamide-imides, which resins have insufficient bond strength to copper foil and still suffer from the problems including lower Tg of cured film and the degraded heat resistance of resin.
In JP-A 2004-051794 or U.S. Ser. No. 10/621,527, the inventors proposed a polyimide silicone having phenolic hydroxyl radicals having high reactivity with epoxy resins, intended for the advanced die bonding or lead frame bonding. It would be desirable to endow the polyimide silicone with more heat resistance and a lower modulus of elasticity.
With respect to the synthesis of polyimide-siloxane copolymer, it is known that if the conventional two-stage synthesis process proceeds by way of polyamic acid of multiple components, the polyamic acid readily undergoes exchange reaction between amide radicals in a solvent, leading to a random copolymer. In the random copolymer, the properties of respective components are averaged. Thus a block copolymer is more effective than a random copolymer in imparting properties of the other component while maintaining the inherent properties of the polyimide resin. There is a need for efficient synthesis of the block copolymer.